The present invention refers to a method and a device for detecting a change between pixel signals which chronologically follow one another, and in particular to a method and a device for detecting moving objects in picture sequences.
Picture sensor elements and the associated electronics represent in various arrangement, e.g. as a one-dimensional array (individual sensors, lines) or as a two-dimensional array, photosensitive integrated circuits (IC) and are implemented e.g. as integrated picture sensors in MOS technology. All photosensitive picture sensors can be used as picture sensor elements, the picture sensor elements implemented in CMOS technology offering a substantial advantage due to the integrability of photosensors and the associated electronics. In CMOS technology, the photodiode, the parasitic, bipolar phototransistor, the photogate and the photosensitive MOSFET (MOS field effect transistor) are known as photosensors. Reading out of the picture sensor elements known from CMOS technology can be carried out by integrated electronics.
The development of intelligent picture sensor elements has led to the result that picture processing algorithms are additionally integrated in the picture sensor elements. Due to the additionally integrated picture processing algorithms, a complex signal analysis of the read picture information with the associated arithmetic expenditure is carried out in the picture sensor element in advance, and a complicated classical picture processing is thus rendered superfluous. The bandwidth of the signal of the picture sensor elements is substantially reduced due to this integration, whereby the real-time ability of an integrated picture processing system is increased. The implemented picture processing algorithms determine the field of use of the integrated picture processing system. They are limited by the circuit techniques of the respective technology.
The use of various picture sensor elements in integrated MOS technology and the integrated electronics for reading the brightness values from the picture sensor elements is known from the prior art. Known picture sensor elements comprise either an individual pixel or a plurality of pixels which are arranged in arrays and which, in response to incident radiation, e.g. incident light, produce a pixel signal indicating e.g. the brightness value of a pixel or of a plurality of pixels. A multidimensional array of the individual pixels is normally referred to as a picture.
The combination of picture sensor elements to form so-called picture sensor networks, the integration of very simple algorithms, e.g. the formation of a difference between two successive pixel signals or pixel values, and the formation of a threshold value are also known from the prior art.
The methods and devices for detecting moving objects in successive pictures known from the prior art have, however, a plurality of disadvantages. The formation of a difference between two successive pixel signals or brightness values, which are pictures in the multidimensional sphere, is an algorithm, which, although it is, on the one hand, easy to integrate, has a high local resolution and entails little additional expenditure for the hardware to be integrated, represents, on the other hand, only the movement between two pixel signals or picture signals taken from pixel signals or pictures which chronologically follow one another, so that small movements cannot be detected. A further disadvantage in the case of difference formation are the so-called artifacts caused by different mechanisms. Such artifacts may be caused e.g. by illumination variations and noise when the picture is being taken, e.g. a temporal noise of the picture sensors, a noise of the A/D converter (analogue-to-digital converter) and of the other circuit elements forming locally restricted large difference values. These artifacts could then be detected as a movement by mistake.
An example of such incorrect detection is shown on the basis of FIG. 6. FIG. 6 schematically shows an algorithm according to the prior art in the case of which a first picture 602 and a second picture 604, or, to be exact, signals representing these pictures, are supplied to a difference former 610 via respective lines 606 and 608. The first picture 602 is the instantaneously detected picture and the picture 604 is the picture detected in the preceding sampling process. The pictures 602 and 610 show, by way of example, a perspective view of a work-room in which a person (at the upper right of the pictures) is present whose movements are to be detected. As indicated by the plus sign and the minus sign at the difference former 610, a difference is formed between the first picture 602 and the second picture 604 so that a signal, which is representative of the difference between the first picture 602 and the second picture 604, will be obtained on the output line 612 of the difference former 610. The signal 612 is inputted in a threshold former 614 having a threshold value S supplied thereto via a line 616. The output line 618 of the threshold former 614 has then applied thereto a binary output signal which is obtained on the basis of the difference-forming algorithm known from the prior art. The binary differential picture is generally designated by reference numeral 620. A movement of the person can be seen in the binary picture 620 at the upper right, the contours of the person are emphasized in comparison with those of the other objects in the room.
Starting from this prior art, it is the object of the present invention to provide a method and a device for detecting a change between pixel signals which chronologically follow one another by means of which movements can be detected more definitely and also small movements can be detected and in the case of which artifacts are prevented from occurring.
This object is achieved by a method according to claim 1 as well as by a device according to claim 9.
The present invention provides a method of detecting a change between pixel signals which chronologically follow one another, the method comprising the steps of:
detecting a pixel signal;
forming a difference between the detected pixel signal and a reference signal; and
determining, in dependence upon this difference, whether there is a change of the detected pixel in comparison with a preceding pixel signal;
wherein the reference signal is produced from at least two pixel signals preceding the detected pixel signal.
The present invention provides a device for detecting a change between pixel signals which chronologically follow one another, the device comprising:
a sensor element producing a pixel signal in dependence upon a radiation power incident thereon;
a subtracter forming a difference between the pixel signal produced by the sensor element and a stored reference signal; and
a judging means judging in dependence upon the difference formed whether there is a change of the detected pixel signal in comparison with preceding pixel signals;
wherein the reference signal is produced from at least two pixel signals preceding the pixel signal produced.
According to a preferred embodiment of the present invention, the reference signal is produced by means of time-recursive calculation.
Preferred embodiments of the present invention are defined in the subclaims.